LOS ANGELES - Los Angeles can continue being the butt of smog jokes now that it has once again topped the American Lung Association's bad air list of most polluted cities in America.

The association found that the Los Angeles metropolitan area, which includes Long Beach and Riverside counties, had the worst air based on 2003 through 2005 figures.

The Pittsburgh area was ranked as the nation's second most polluted metropolitan area followed by Bakersfield, Calif., Birmingham, Ala., Detroit and Cleveland. Visalia, Calif., Cincinnati, Indianapolis and St. Louis rounded out the top 10.

The news wasn't all bad for Los Angeles. Despite the dubious distinction, the number of days residents breathed the nation's worst ozone levels was fewer than in previous years.

"Nobody is surprised that LA has an air pollution problem," said Janice Nolen, the association's assistant vice president for national policy and advocacy. "The problems there are one of the reasons we have the Clean Air Act. But it is important for folks to know that there has been some improvement."

The organization based the rankings on ozone pollution levels produced when heat and sunlight come into contact with pollutants from power plants, cars, refineries and other sources. The group also studied particle pollution levels emitted from these sources, which are made up of a mix of tiny solid and liquid particles in the air.

Such pollution can contribute to heart disease, lung cancer and asthma attacks, the association said. Those especially vulnerable to polluted air are children, senior citizens, people who work or exercise outdoors and people with asthma or chronic obstructive pulmonary disease.

Nearly half of the U.S. population lives in counties that still have unhealthy levels of ozone or particle pollution, even though there appeared to be less ozone in many counties than previous years, the study found.